You don't need a 2-country pass as you will only be doing a single trip in France. You can buy a separate ticket to to Basel (the border) from the websites listed here. From Basel, the Swiss Half Fare Card or any other Swiss train pass will work. You can print the card at home to save time (see here). Tickets can then be purchased from the machines or the staffed ticket office. Prices of such tickets are the same online or on the spot.

Which pass is best depends on what your plans are in the Interlaken area. Please see here for how to choose.